1. Field of the Invention
The present invention relates to a piston ring for use in an internal combustion engine or for a reciprocating piston of a compressor, a shock absorber or the like. More particularly, it is concerned with a piston ring made of a sintered alloy.
2. Description of the Prior Art
As illustrated in FIG. 1, since a piston ring 3 slides on the cylinder wall with the axial movement of a piston, an outer peripheral surface 31 of the piston ring 3 is required to have good abrasion resistance. Particularly, in the case of a piston ring for use in an internal combustion engine, a bottom surface 32 of the piston ring 3 is required to have good abrasion resistance since it is pressed onto a groove 4 by the pressure in the combustion room.
With one type of conventional piston ring, a hard chromium plating or sprayed coating of abrasion resistant material is applied all over the outer and inner peripheral surfaces of a parent material of cast iron as well as all over the bottom and top surfaces thereof. This surface coating is applied after the piston ring is molded. Therefore, the number of working steps and the time necessary for working this conventional piston is increased.
As far back as the nineteen-fourties, a piston ring formed of sintered alloy was developed in England. Since that time, various types of sintered alloys have been employed in piston rings.
One conventional piston ring is made entirely of a parent material which comprises a sintered alloy having good abrasion resistance. Generally, as disclosed in Japanese patent publication No. Sho 45-32803, a single kind of powder is used for molding and sintering. In this latter type of piston ring, the number of working steps and working costs is reduced. However, the starting materials required for producing the high abrasion resistant parent materials are expensive. Furthermore, these high abrasion resistant sintered alloys have inferior strength because of the high carbon content thereof. Therefore, these piston rings require additional surface treatments such as infiltration and sulfurizing.
In order to overcome the above described defects of piston rings made of sintered alloys, a piston ring having different material layers have been proposed.
Composite techniques and methods have been developed wherein piston rings have a plurality of layers. Because of the composite structure, the piston rings function well and the cost for the raw materials for these piston rings is reduced.
One such composite sintered alloy piston ring is attained by stacking a plurality of powder layers when molding the powders. According to U.S. Pat. Nos. 2,753,858 and 2,753,859, after a first powder is pressurized and molded. a second powder is filled over the first pressurized powder layer. Making piston rings using this technique is complicated.
For example, a composite piston ring as illustrated in FIG. 2 is known. The piston ring of FIG. 2 comprises a second sintered alloy layer 2 having excellent abrasion resistance and a first sintered alloy layer 1 made of a usual low alloyed metal which is laminated on the second sintered alloy layer 2. In this composite piston, however, the volume of the second sintered alloy layer 2 is large which still results in rather large material costs. Furthermore, the first sintered alloy layer 1 is required to have a high density to maintain sufficient strength over the operational life of the piston ring. Therefore, the first sintered alloy is made of a low alloyed metal. On the other hand, the second sintered alloy layer 2 is required to have a low density so that lubricating oil can enter voids existing in the second layer 2 to increase the abrasion resistance thereof. Therefore, as the volume of the second layer 2 is increased and the volume of the first layer 1 decreased, the strength of the piston ring is reduced.
Further, as another example of a complicated technique for making sintered alloys, Japanese patent publication No. Sho-51-39166 discloses a method for producing a sintered alloy having multiple layers by dividing a die into a plurality of pieces. This technique can be applied to make a variety of mechanical parts such as those shown in Japanese patent publication No. 54-31963 and Japanese patent application publication (OPI) No. 54-23810. In publication No. 51-39166, the powder layers for sintering are not only arranged vertically or horizontally, but also arranged by forming different powder material layers at specific portions of the mechanical parts such as at corners to enchance the mechanical properties of the mechanical parts and to reduce the cost of the raw materials required. However, according to the prior art, a plurality of punch means are required to make such complicated structures and therefore the punching strength of the press machine is weak and the operational sequence followed to make the mechanical parts is complicated. As a result, the dimensional accuracy of the resultant mechanical part is low. Oftentimes, the mechanical part thus formed is not suitable for being put into practical use.